1. Field of the Invention
The present invention relates to a method controlling backlight of portable electronic device, especially to a method for controlling backlight of portable electronic device in hysteresis manner.
2. Description of Prior Art
Portable electronic devices such as mobile phone and personal digital assistants (PDAs) have enhanced function and gained popularity as technology keeps progressing. Moreover, user has more chance to watch the display of the portable electronic devices as the price of LCD display is reduced and LCD is omnipresent for portable electronic devices.
One of the major requirements for LCD screen used in portable electronic devices is adaptation for environment brightness because the portable electronic devices are usually used in mobile manner. The backlight of the LCD screen of the portable electronic devices needs adjustment for saving power and providing comfortable reading. When the environment brightness is intense, the backlight of the LCD screen of the portable electronic devices had better turn off and the reading illumination relies on light reflected from the LCD screen. When the environment brightness is not sufficient, user feel uncomfortable in reading the LCD screen if the backlight of the LCD screen of the portable electronic devices is much brighter than the environment brightness.
Depending on the position of light source, the LCD screen can be, for example, transmissive display or reflective display. The transmissive LCD screen uses a light source at back side thereof and user watches the LCD screen from the front side. The transmissive LCD screen is generally used for high-brightness application such as computer display. The reflective LCD screen uses light reflected from a reflecting face at back side thereof as light source, and is often used for electronic watches or PDA. Moreover, transreflective LCD screen can be used as transmissive display or reflective display. More particularly, the transreflective LCD screen is used as reflective display when environment brightness is ample, and the transreflective LCD screen is used as transmissive display when environment brightness is weak.
A prior art LCD screen with brightness adjustment mechanism is developed for adaptation to environment brightness. The brightness adjustment mechanism comprises a light sensor and a backlight adjustment unit. FIG. 1 shows a backlight adjustment curve for in the prior art LCD screen with brightness adjustment mechanism, where the LCD screen is a transreflective LCD screen. The environment brightness sensed by the light sensor is divided into three regions defined by thresholds C1 and C2, namely, the weak brightness region, the normal brightness region, and the high brightness region. The backlight adjustment unit adjusts the backlight brightness of the LCD screen with reference to the three regions defined by thresholds C1 and C2. However, the environment brightness may change at the vicinity for one of the thresholds C1 and C2 if the user is mobile. For example, when the LCD screen is for PDA used by a parking metering staff, the environment brightness may change as the staff is moving. As a result, the backlight brightness of the LCD screen changes rapidly as the staff is moving. This causes user's eye fatigue and frequent work of the backlight adjustment unit.